Deflector for converting a beam of parallel rays into a beam of rays having constant incidence on cylindrical part

ABSTRACT

A deflector for exciting Lamb waves at the surface of a cylindrical part having a polygonal or circular directrix converts a beam of rays parallel to the generating-lines of the part into a beam of rays which impinge upon the surface at a constant angle of incidence in a family of broken helices. The deflector comprises a plurality of flat deflecting elements each associated with one face of the part and having an orientation defined by the direction of a normal. In order to produce excitation in phase of the Lamb waves from one face to another on the concurrent segments od a broken helix, the planes of two flat deflecting elements are deduced from each other by a movement of rotation about a parallel to the generating lines followed by a movement of translation parallel to the generating-lines of the part.

United sw X Wm Perdijon DEFLECTOR FOR CONVERTING A BEAM OF PARALLEL RAYS INTO A BEAM OF RAYS HAVING CONSTANT INCIDENCE ON CYLINDRICAL PART Inventor: Jean Perdijon, St-lsmier, France Assignee: Commissariat a IEnergie Atomique,

Paris, France Filed: Dec. 23, 1974 Appl. No.1 535,703

Foreign Application Priority Data Dec. 26, 1973 France 73.46384 U.S. Cl. 73/67.8 R; 73/67.8 S Int. Cl. GOIN 29/04 Field of Search 73/67.7, 67.8 R, 67.8 S,

73/679, 71.5 US, 67.5 R, 67.6

References Cited FOREIGN PATENTS OR APPLICATIONS 1/1961 U.S.S.R 73/67.8 R

3/1968 U.S.S.R

3/1969 Switzerland 73/67.5 R

Nov. 4, 1975 Primary Examiner-Richard C. Queisser Assistant Examiner-John P. Beauchamp Attorney, Agent, or Firm-Lane, Aitken, Dunner & Ziems [57] ABSTRACT A deflector for exciting Lamb waves at the surface of a cylindrical part having a polygonal or circular directrix converts a beam of rays parallel to the generatinglines of the part into a beam of rays which impinge upon the surface at a constant angle of incidence in a family of broken helices. The deflector comprises a plurality of flat deflecting elements each associated with one face of the part and having an orientation defined by the direction of a normal. In order to produce excitation in phase of the Lamb waves from one face to another on the concurrent segments 0d a broken helix, the planes of two flat deflecting elements are deduced from each other by a movement of rotation about a parallel to the generating lines followed by a movement of translation parallel to the generatinglines of the part.

10 Claims, 11 Drawing Figures US. Patent Nov. 4, 1975 Sheet 1 of8 3,916,675

US. Patent Nov. 4, 1975 Sheet 2 of8 3,916,675

FIG.3

US. Patent 7 Nov. 4, 1975 Sheet 3 of8 3,916,675

QQI

U.S. Patant Nov. 4, 1975 Sheet 5 of 8 3,916,675

U.S. Patent Nov. 4, 1975 Sheet 6 of8 3,916,675

FIG.8

US. Patent Nov. 4, 1975 Sheet 7 of 8 3,916,675

US. Patent Nov. 4, 1975 Sheet 8 of8 3,916,675

FIGJO IMZK LEETQE IFQR CQNVIEIRTING A BEAM OF IWQRA LLIEL llhl'lfl A WEAR/ll OIF RAYS HAVING CUNSTANT INCIIIDIZNQE ON (IIiLllNDlRHCAL PART This invention relates to a deflector in which the shape of the deflecting surface is such that a beam of parallel rays emitted by a radiation source and deflected by the deflector impinges upon a cylindrical part at a fixed and constant angle of incidence, said angle being measured with respect to the normal to the lateral surface of the cylindrical part at the point of impact of the ray.

The invention relates in particular to an ultrasonic deflector for reflecting the rays which are emitted by an ultrasonic transducer and intended to impinge upon a cylindrical part and to excite waves therein; said waves propagate within the cylindrical material and are diffracted by the flaws which are present in the part and modify the propagation of the diffracted wave; said diffracted wave is received by the transducer which performs the function of a receiver and thus makes it possible to detect the positions of said flaws.

The deflector in accordance with the invention finds a preferential application in the deflection of ultrasonic rays but is also applicable to the deflection of all electromagnetic or corpuscular rays.

It is known that the object of ultrasonic detection is to study the properties of materials by means of ultrasonic waves, that is to say waves having frequencies above the upper limit of the audible range (approximately 16 kilocycles per second). Ultrasonic detection entails the use of measurements either of velocity or of attenuation. Ultrasonic detection of flaws is usually carried out by propagating ultrasonic wave trains through the material or by inducing ultrasonic vibrations in the material and collecting the echos which resuit from the presence of said flaws.

A technique which is commonly employed for solid tubes having cylindrical shapes of revolution consists in using two transducers whose movements are coupled for helical scanning; these two transducers are placed in a tank filled with water:

one transducer is located in a meridian plane of the tube and its axis is inclined with respect to the normal in order to carry out preferential detection of flaws having an orientation at right angles to the axis of the tube; these flaws will be designated hereinafter as transverse flaws,

the other transducer which is located in an equatorial plane has an axis which is not concurrent with that of the tube in order to carry out preferential detection of flaws having an orientation parallel to the axis of the tube (longitudinal flaws).

In the case of thin tubes or plates, it is known that the mode of propagation of ultrasonic waves which is best suited to the detection of flaws is the Lamb mode. Lamb waves are waves which propagate parallel to the surfaces of the material: the entire thickness of the material begins to vibrate and this permits detection of flaws independently of the depth at which these latter are located. In point of fact, these waves are modes inherent to the geometrical configuration in which they are generated and can be excited in a material of given thickness and at a given frequency only in respect of well-determined angles of incidence of the exciting ultrasonic waves.

Thus in the case of a cylindrical stainless steel tube having a thickness of 0.5 mm and in the case of a piezoelectric disc which emits ultrasonic waves having a frequency of 4 Mc/s, the angle of incidence of excitation of the Lamb waves corresponding to the fundamental anti-symmetrical mode A, (which is the most suitable in this case) is equal to 34. Only the rays which fall on the tube at this angle of incidence will be capable of generating Lamb waves within this tube; flaws can be detected by this means since said waves are propagated differently according to whether flaws are either present or absent in the tube.

The flaw is detected by measuring the time interval which elapses between the emission of a wave train and the return of the reflected wave through the flaw or alternatively by placing a second transducer-receiver for receiving the waves which are reflected or diffracted by the flaw.

In order to ensure sufficiently high intensity of these Lamb waves, there is a second condition to be satisfied in addition to the first condition relating to the angle of inclination of the incident rays: the number of periods of Lamb waves in phase with the longitudinal waves of the incident beam must be at least equal to 4 or 5 in order to ensure that the Lamb waves within the tube are sufficiently strongly excited. The angle of divergence of the beam within which the angle of incidence is very close to the angle corresponding to the generation of the Lamb waves of the mode selected must therefore be as large as possible.

The achievement of these conditions does not give rise to any problem in the case of detection of transverse flaws: as shown in FIG. 1, if the section of the disc constituting the transducer along the plane defined by the axes of the tube and of the transducer is a segment of straight line of length a having an angle of inclination equal to the angle 0 with respect to the surface of the cylindrical tube, the incident rays in this plane have a constant angle of incidence 0 and the waves are in phase over a length of tube equal to a/cos 0; on the other hand, the two conditions are more difficult to obtain in the case of longitudinal flaws since it is then necessary to ensure that the rays impinging on the tube and located in a plane at right angles to the axis of the tube make a constant angle with the circle, namely the cross-section of the cylinder along said plane.

The first transducers employed for the detection of longitudinal flaws in tubes were flat. In this case, the mean ray emitted by the center of the transducer impinges on the cylinder at an angle 0 but the two end rays derived from the edges of the transducer make different angles with the normal to the cylinder at the point of impact as will be indicated more precisely in FIG. 2 which will be described below. Thus the arc of circumference on which the waves are in phase is very small and the Lamb waves are weakly excited.

Use has also been made of transducers associated with cylindrical discs or lenses having circular directrices. Although this system offers advantages over the flat lensless transducer, the angle of incidence of the rays as measured with respect to the normal to the cylinder varies within the beam.

Finally, it is not possible by means of any device of the prior art to direct rays having a constant angle of incidence along a helix formed on a cylinder, especially with a view to exciting waves within the material along said helix which is either a broken helix formed by a plurality of segments having an equal angle of inclination with respect to the generating-lines of the prismatic cylinder or a continuous helix having a constant angle of inclination with respect to the generating-lines of a cylinder having a circular directrix. The excitation of the waves along a helix of this type permits fully efficient detection of flaws which are inclined with respect to the generating-lines of the cylinder at an angle which is complementary to that of said helix. The detection along a helix is particularly useful when tubes are welded along a plane which is inclined with respect to the generating-lines of the cylinders constituting said tubes; in this case, the angle of the helix with respect to the generating-lines is chosen so as to be complemen tary to the angle made by the inclined plane with the same generating-lines.

The precise aim of the present invention is to provide a deflector which is primarily intended to excite Lamb waves at the surface of a cylindrical part C, the cross section of said part being either a polygonal or circular directrix.

The deflector converts a beam F of rays which are parallel to the generating-lines of the cylindrical part C having a polygonal directrix into a beam F of rays which impinges upon the surface of said part in a family of broken helices which are parallel to each other, each helix being constituted by a plurality of straight-line segments which are concurrent in pairs along one edge of the cylindrical part C. The rays of the beam F make a constant angle of incidence i with respect to the normal to the surface of the part at the point at which each ray of the beam F strikes said surface. Each segment of the helix is inclined at an angle ,8 with respect to a plane at right angles to the generating-lines of said cylindrical part. It is postulated that said angle [3 is constant from one face to the other in the deflector in accordance with the invention. However, by making minor changes, it would be possible to vary said angle from one face to the other although the technical advantage of this is not at present apparent. Since the angle [3 is chosen so as to have a fixed value, the broken helix formed by the plurality of segments aforementioned is a helix having a constant pitch Each flat deflecting element M, is associated with one face P, of said part C. The orientation of each flat deflecting element M, is defined by the direction of the normal r7, having projections which are proportional in three perpendicular directions and which are a generating-line of the part C, a normal to the face P, and an axis at right angles to the two preceding projections respectively at cos a, cos j sin a, and sin j sin a. The anglesj and a are defined from the angles 1' and ,3 by the relations tan j= tan i cos [3 and cos 201 sin 1' sin ,8. The orientation of these deflectors makes it possible in the case ofa beam F of rays which arrive parallel to the generating-lines of the cylindrical part to deflect the rays in a beam F of constant incidence upon the lateral faces of the cylindrical part C. In order to ensure that the condition of excitation in phase of the Lamb waves is satisfied from one face to the other on the concurrent segments of a broken helix, two flat deflecting elements M, and M associated with two faces P, and P of the part C are deduced with respect to each other by a movement of rotation and a movement of translation. The rotation takes place about an axis parallel to the generating lines of the part C and located in a plane which is equidistant from the edges A, and A the angle of said movement of rotation is equal to the angle of the dihedron constituted by the faces P, and P and there is added to this movement of rotation a movement of translation which is parallel to the generating-lines by a quantity a sin j tan where a is the width of the face P,. As will become apparent hereinafter, the result thereby achieved is that rays which arrive on the edge A, which is common to the two faces P, and P and are derived from two consecutive deflectors are of equal phase.

In an alternative embodiment, the deflector in accordance with the invention is such that the deflected rays of the beam F are incident upon the cylindrical part C in a direction at right angles to the generating-lines and make a constant angle i with the normal to the face of the part C. In consequence, the families of broken helices are reduced to a family of parallel polygonal directrices located in an equatorial plane at right angles to the generating-lines. This corresponds to B=0 and consequently to a 1r/4 In this case, the deflector is constituted by a plurality of flats deflecting elements, each of these deflectors M, being associated with one face P, of said part, the section of each deflecting element M, aforesaid through a plane at right angles to the generating-lines of the part C being a segment of straight line 8,. The deflecting elements M, are disposed in such a manner as to ensure that each segment S, makes the same angle i with the associated face P,; moreover, two of these segments S, and S, which form part of two consecutive deflectors M, and M, are equidistant from the edge A, located at the intersection of the two faces P, and P In another alternative embodiment of the invention in which the angle B defined earlier is equal to IT/2, the families of broken helices are reduced to segments of generating-lines. In this case, the deflector is constituted by a plurality of flat deflecting elements M,, each deflecting element M, which is associated with the face P, being such as to make an angle with the associated face P,

Longitudinal flaws are preferentially detected in the case in which ,8 is equal to zero whereas flaws having an orientation at right angles to the axis of the tube or in other words transverse flaws are preferentially detected in the case in which B is equal to 11/2 by exciting in both cases waves having a direction of propagation at right angles to the direction of said flaws.

It is an advantage in all cases to ensure that the rays arrive with a constant density or in other words that the intensity of incident radiation is constant per unit of cylindrical surface irrespective of the generating-line considered. The fact that the rays arrive with a constant density means that there are no preferential faces of the polygon or no angular sector which is more especially illuminated by the rays. In fact, in the case of excitation of Lamb waves, it is desirable to ensure that the level of excitation of these waves which are generated within the material as a result of the cumulative effect of a number of deflectors which direct rays along one and the same helix attains the same value irrespective of the generating-line considered in order to permit detection of flaws in the material in any angular position, that is to say irrespective of the generating-lines on which the flaw is located. In order to achieve this result in the case in which B 0, each plane at right angles to the generating-lines intersects the deflector along segments 8,, the cumulated length of which is constant irrespective of the plane chosen.

In the case in which the cylindrical part C has a circular section or in other words has a symmetry of revolution with respect to the axis of the right cylinder which constitutes the part, the deflector is constituted by a continuous surface whic h can be considered as the limiting surface of the different planes of the deflecting elements considered earlier; this limit is obtained when all the sides of the polygon constituting the directrix are ,the number of said deflecting elements tends towards infinity. This transition to the limit shows that the surface S is ruled and developable, which is an advantage since it can readily be formed by milling. As in the previous instance, the angles a and g are related to the angles i and [3 by the relations tan j tan i cos ,B and cos 2 1= i sin i sin B. The axis 02 coincides with the axis of the cylindrical part.

As in a previous alternative embodiment, if it is desired to ensure that the rays of the beam F are contained in an equatorial plane, that is to say a plane located at right angles to the axis of the cylinder, the angle B defined earlier is zero and the helices are replaced by directorcircles. The surface of the deflector then corresponds to the'equation expressed in polar coordinates havin anaxis Oz To insure a constant illumination of all the generating-lines of the cylinder the intersection of the surface of the deflector along the planes at right angles to the generating lines are are of involutes (of circles) of constant length.

in order to match the impedanc'es, it is an advantage to interpose a couplant liquid such as water for example between the transducer and the surface to be inspected. Moreover, for the excitation of the Lamb waves especially in cylinders of small thickness, the transducer and the deflector can be placed inside or outside the cylinder. In the event that the reflector and the transducer are placed outside the volume delimited by the external surface of the cylinder, the transducer is in that case an annular transducer which is coaxial with the cylinder. It is readily apparent that, if the flat deflecting elements have tov be oriented as indicated by the formulae given above and the shape of the surface of the deflectors is intended to come as close as possible to the formula expressed in polar coordinates, said deflectors must be givena practical construction; this necessarily results in small'differences between the ideal geometrical configurations and the practical constructions without thereby resulting in any departure of these latter from the scope of the invention. 7

Further properties and advantages of the invention will become more readily apparent from the following description of examples of application which are given by way of explanation'and not in any limiting sense, reference being made to the accompanying figures, wherein:

FIG. 1 shows a flat transducer which directs a beam 'onto a surface at an angle corresponding to the excitation of Lamb waves;

' FIG; 2 shows the variation of the angle of incidence in the case of a flat transducer which directs a beam of parallel rays onto a circle;

FIG. 3 shows the variation in the angle of inclination of the rays which impinge on a circle, said rays being focused by a disc or a lenshaving a circular directrix;

.FIG. 4 shows the geometrical parameters which are related to the travel of the incident rays along a broken helix on a prismatic cylindrical part, these geometrical parameters being such as to define the positions of the flat deflecting elements;

FIG. 5a and 5b show a deflector which is composed of a plurality of flat deflecting elements and intended to deflect a beam F of rays parallel to the generating-lines of a right cylinder having a base in the shape of a regular hexagon so as to form a beam F of rays having a constant incidence on a transverse section;

FIG. 6 shows a deflector which is composed of six flat deflecting elements and is intended to deflect a beam F of rays parallel to the generating-lines of a right cylinder having a base in the form of a regular hexagon in a beam F of rays having a constant incidence on a generating-line of said cylinder;

FIG. 7 shows the geometrical parameters which are related to the travel of incident rays along a continuous helix on a cylindrical part having a circular directrix, these parameters being intended to define the geometrical nature of the surface constituting the deflector;

FIG. 8 shows an outline of a deflector in the form of a helix for deflecting a beam F of rays parallel to the generating-lines of a cylinder having a circular directrix in a beam F of rays located in an equatorial plane and having a constant incidence on a directrix of the cylinder;

FIG. 9 shows an assembly consisting of a deflector in the form of a helix, a transducer and a cylinder in the case in which the transducer and deflector are located inside the cylinder;

FIG. 10 is a perspective view of a helical deflector obtained by milling in the mass.

There is shown in FIG. 1 the diagram of a known device for inducing Lamb waves on a plate 2. The transducer 4 of diameter a has an angle of inclination 6 with respect to the plane constituted by the top face of the plate 2. Said transducer emits a beam of parallel rays in which the wavelength is equal to M. The rays of the beam of waves make an angle 6 with the normal to the surface of the plate 2. The Lamb waves in phase with the incident waves having wavelengths A, are such that M M/sin 6; a Lamb wave is shown at 6. In this embodiment, the Lamb waves are excited in phase over a length equal to a/cos 6.

There is shown in FIG. 2 a transducer 4 for emitting a beam of parallel rays received by a thin tube in which the cross-section of the external surface has the shape of a circle C having a radius R and a center 0. It is ap- Lamb waves within the cylinder: in other words, the coupling between the transducer and the medium constituted by the tube of section C is poor.

There is shown in FIG. 3 a device for focusing waves on a cylinder having a circular section C with a view to reducing the variations in angle of inclination of the incident rays with respect to the external wall of said cylinder. Said focusing device comprises a cylindrical disc or lens of circular section, the focal line of which passes through the point N and is parallel to the axis of the cylinder whose section is the circle C. The angle a is the semivertical angle of the beam which is convergent on the point N. The rays coming from the extremities of the disc or lens L intersect the circle C at A and B. The points A, B, N and O are located on a circle. Since the focal point N of the beam is located on the circle defined by the center of the tube and the end points of impact A and B, the angles of incidence at A and B are equal to 6 e. The incidence is not constant in this known system. As the arc AB is of greater length in order to increase the length of excitation of the Lamb waves on the wall of the cylinder (second condition), so the variation in the angle of incidence is greater and this is contrary to the essential requirements of the first condition of excitation of Lamb waves.

The geometrical parameters which define the deflector in accordance with the invention are shown in FIG. 4. A cylindrical face C of the part contains the axes Ox and Oz, Oz being on the edge A of the face P,. This face P, is a surface of the cylindrical prismatic part C having a polygonal directrix. The plane xOy is perpendicular to the generating-lines of the part C and to the edges such as Oz. It is desired to excite waves in a family of broken helices having an angle of inclination B with respect to the plane at right angles to the generating-lines, that is to say the plane xOy; said helices are constituted by a plurality of contiguous segments, a single segment TV being shown in this figure. The beam F of parallel rays emitted by a transducer, not shown in this figure, is a beam F of rays which are parallel to the generating-lines of the part C. The ray PM is one of these rays. The point M is the point of the deflector at which the ray PM is reflected along the line MA. Tiis the normal to the surface of the deflector at the point M, the angles a being the angles of incidence and of reflection on said deflector. The ray MA makes a constant angle i with the normal ii to the face P at the point A located along the segment TV. The point N is the projection of the point M on the plane xOy; similarly, the point B is the projection of the point A on the same plane. The segment KMJ represents the intersection along a horizontal plane parallel to the plane xOy which passes through the point M of the deflector M, which is associated with the face P. This intersection KM] is projected along KNJ' on the plane xOy. The point E is the projection of the point A on the segment MN; the point C is the projection of the point M on the normal 77 to the surface of the cylinder at the point A. By virtue of elementary geometrical considerations, all the angles indicated by the same denominations such as 1 ,1, B, 51 are equal angles. In fact, the anglejbetween K'NJ' and the axis Ox is equal to j since it is equal as an acute angle having one side perpendicular to the angle NE? (88' is the normal to the axis Ox at the point B). As a result of the geometry of the traingles AMC, AME, AEC and MCE, the angles i j, a and 3 are related by the relations: tanj tan icos B and cos 2a=isin Bsin i. It can readily be seen from this figure that the anglej between the face P and the section KMJ of the deflector in a plane at right angles to the generating-lines is a constant angle given as a function of i and B by one of the formulae which appeared earlier. Similarly, the flat surface of the deflector M could have been characterized by another of its sections, namely the section WMZ of the deflector in the plane of incidence of the rays on the deflector, that is to say the plane defined by the straight line PM and the normal fito the deflector. It is readily apparent that the angle of said section WMZ in the plane of incidence as defined makes an angle (17/2 a) with the axis Oz, that is to say with the direction of the generating-lines of the cylindrical part. Elementary geometrical calculations show that the normal fito the deflector at the point M has Oz: director cosines along the three axes Ox, Oy and Ox: sin a sin j, sin a cos j, cos 0:; since the angles i and B are known, the value ofa is calculated by means of the relation cos 2oz= i sin B sin i and the value ofj is calculated by means of the relation tan j tan i cos B. The three director cosines of the normal i are then known with accuracy and this establishes at absolute value the angle of inclination of the flat deflector which passes through the point M. As will also be explained hereinafter, the inclination of the plane of the deflecting element being determined, the position of said element is also determined by satisfying the condition which ensures that the incident rays from two consecutive deflectors on one point of an edge such as Oz are in phase.

The mirror M is deduced from the mirror M; by a rotation about an axis parallel to the generating-lines through an angle equal to that of the dihedron constituted by the faces P, and P followed by a movement of translation of the vector which is parallel to Oz and has a value OT sin j tan a. The axis of rotation which causes the edge A, to coincide with the edge A is contained in the plane which is equidistant from the edges A and A In FIGS. 5a and Sb, there is shown a deflector associated with a cylindrical prismatic part C having a regular hexagon directrix. FIGS. 5a and 5b correspond to the particular case in which the rays such as 24 deflected by the deflector are in a plane at right angles to the generating-lines of the cylindrical part C. FIG. 5b shows the deflector and the cylindrical part C which is displaced in the vertical direction for reasons of clarity of the drawing. The deflector comprises a plurality of deflecting elements D D D D D D D and D Consideration is given to the rays of the beam F which fall on the portions of the deflector C B B A A F and which are reflected from the surface of said deflectors in an equatorial plane in a beam F. The rays such as the ray 26 of the beam F which are contained in an equatorial plane make an angle i with the associated face of the cylindrical part as shown in the figure. It can be verified that the segment A 3 which is a section of the transducer D in an equatorial plane of the cylinder makes an angle i with the face of the associated cylindrical part C, said angle i being represented by the angle between the extensions of the segments A 8 and A B The deflecting elements D D etc. are connected by means of planes such as those designated by the references 30, 32 and so forth which are not essential to good operation of the deflector in accordance with the invention but are readily obtained by folding the strip which constitutes the deflector on a machined former in which the planes constituting the deflecting planes D D etc. are oriented in a precise manner. The rays of the beam F which fall on the segments C 8 B A A F are reflected from the part C in an equatorial plane and excite waves along the segments C B B A A F The edge which passes through A is equidistant from the points A and A with the result that the waves excited at A by the two deflecting elements D and D are in phase. Similarly, the lengths B B, and B B are equal, with the result that the waves are excited in phase at the point B by the two deflectors D and D This phenomenon is general: the difference in path between two incident rays at any two points on the surface of the part C corresponds exactly to the difference in phase of the wave induced in the material between the two points at which the rays meet the tube. In consequence, the waves are in fact excited in phase by all the rays which come from the transducer and are deflected by the deflecting elements.

It is now necessary to show that the waves which are excited at any point of the object have the same value of excitation. The waves which are excited at F result from the cumulative effect of the incident rays on the segments C B B A A F The rays which are directed to these three segments come from the rays of the beam F which are deflected by the segments of the deflectors C 8 B A and A F- The level of excitation of the Lamb wave at the point F 0 will thus be proportional to the cumulated length of the segments C 8 B A and A F namely The deflector is chosen so as to ensure that the cumulated length of the intersections of the deflecting elements in all the planes at right angles to the generatinglines of the different elements of the deflector is constant. Thus the level of excitation at any point of the directrix A B C D E F corresponds to excitations produced by segments of deflectors having a constant cumulated length, with the result that said excitation level is constant.

In the case of FIGS. a and 5b, the angle ihas a value of 30 and the deflector is employed for observing longitudinal flaws, that is to say flaws which are parallel to the axis of the part C. The transducer T is shown in chain-dotted lines at the top of FIG. 5b and is intended to deliver a beam of rays F. The intersections of the deflecting elements D D and so forth in an equatorial plane are relatively displaced so as to ensure that the paths between a given edge and the two facets which illuminate said edge are equal for the continuity of phases from one face to the other. The pitch of the pseudohelix constituted by the different deflectors D D D D D D D D is equal to 6 a sin i, in which, as indicated in FIG. 5a, a is the width of one of the sides of the hexagon constituting a section of the part C in an equatorial plane. It can be established that any two consecutive deflecting elements such as D, and D for example .are deduced from each other by a movement of rota tion through an angle of 60 about the axis of the hexagonal cylinder which passes through 0, followed by a movement of translation parallel to the generating-lines of the value a sin i. All the deflecting elements will thus have been obtained, that is to say the pitch of the helix 10 considered, after a movement of translation of 6a sin 1' which is the pitch of the helix.

It is readily understood that the transducer T and the deflector are usually stationary and that the part C can be observed continuously by displacing said part in translational motion along its axis within the stationary transducer-deflector unit. It is thus possible to observe the entire surface of a tube of prismatic section, the transducer T being intended to perform the function of an emitter-receiver while displacing said part in translational motion.

FIG. 6 shows a deflector in accordance with the invention for observing transverse flaws in a cylindrical part C having a section in the shape of a regular hexagon. The beam of rays F emitted by the transducer T is reflected from the elements of the deflector such as the elements P and P and impinges on the surface of the part C at a constant angle of incidence 1, namely 30 in the case of the figure. The rays of the beam F are in phase along the same generating-line of the part C.

All the generating-lines of the part C are subjected to the same intensity of radiation since the length of the segments A"B" is constant irrespective of the section of the elements of the deflector in planes parallel to the generating-lines of the part C and at right angles to the faces.

The deflector which has just been considered serves to observe transverse flaws in a part having a section in the shape of a regular hexagon. As in FIG. 5, it can be established that all the points of the generating-lines are excited with the same amplitude, the illumination of the generating-lines being constant at each point. The deflectors form a frustum of a regular pyramid having six sides. Each side makes an angle with the associated face of the prismatic part C of hexagonal section. The sign 1- in the expression which defines the angle depends on whether it is desired to cause the beam of rays F to impinge on the deflector at an acute or obtuse angle 2a; the rays of the beam F travel upwards with respect to the horizontal in the case of an acute angle 201- as shown in the figure whereas said rays travel downwards with respect to the horizontal in the case of an obtuse angle 20:.

FIG. 7 shows the geometrical parameters which are related to the path of the incident rays along a circular and continuous helix H on a cylindrical part C having a circular directrix. The geometrical parameters such as i the angles i, j, at and B define the geometrical nature of the surface which constitutes the deflector. The ray PM which is parallel to the axis Oz of the cylinder constituting the part C is incident at M on the surface of the deflector and is reflected along the segment MA which makes an angle of incidence iwith the normal N to the cylinder C at A. The point M is projected along a vertical line parallel to the generating-lines at N on the plane xOy and it can be verified that the projection of the normal Won the plane xOy constituted by the segment NB makes a constant angle j with the normal to it it the circular directrix of radius R of the cylinder C. To give a more geometrical definition of the surface of the deflector, the projections of the normals to the surface of the deflector such as fion a plane at right angles to said generating-lines enclose a circle having a radius R sin j (the circle which passes through F in FIG. 7). The rays are incident on the helix H which passes through the point A and is represented by a line of double thickness. The tangent to said helix at any point A makes an angle B with the plane at right angles to the generating-lines, that is to say the plane xOy. The properties of the projections to the normal ii'to the deflector on the plane xOy which enclose the circle having a radius R sinj are such that the section of the surface of the deflector along said plane is an involute of the circle having a radius R sin j, j, being related to the angles i and B by the relation tan j tan i cos [3.

FIG. 8 shows the deflector M having an equation defined by means of its rectilinear generating-lines in the case in which said deflector M surrrounds the cylinder 40 of radius R. The surface of the deflector M is a ruled surface. A first rectilinear generating-line 52 extends from the point A located on the circle 54 having a radius R sin j. Said generating-line is located in the plane 56 tangent to the cylinder in which the cross-section at the base of said cylinder is the circle 54. The generating-line 52 makes an angle a (in this case of figure a 1r/4 with a plane at right angles to the axis Oz. A second generating-line such as the line 58 corresponding to a movement of rotation of 77/2 starts from a point B located on a generating-line of the cylinder having a radius R sin j at a distance from the plane containing the point A which is equal to 1rRsinjtanrx This generating-line is contained in a plane tangent to the cylinder of radius R sin j and of axis Oz and makes an angle a with a plane at right angles to the axis Oz. The generating-lines 60, 62 and 64 are obtained in the same manner. The generating-line 64 is obtained from the point C located on the cylinder of radius R sin j, the segment AC being parallel to the axis Oz and having a length 27rR sinjtan oz. Said generating-line 64 is parallel to the generating-line 52. In FIG. 8, [i= and a= 45 have been chosen, with the result that the rays deflected by the deflector M impinge on the part 40 in a plane at right angles to the generating-lines and that the helices H are reduced to director circles.

FIG. 9 shows the deflector M and a transducer 66 in the case in which said deflector M and the transducer are placed inside the cylinder 68 of internal radius R. The rays emitted by the transducer in a direction parallel to the axis Oz are reflected by the mirror M and strike the internal surface of the cylindrical tube 68 at a constant angle of incidence equal to i. Said rays are shown at 72, 74 and 76.

FIG. is a view in perspective showing the deflector M of FIG. 8 in the case in which said deflector is milled in the solid. The surface corresponding to the deflector M is represented at 200 and the cylindrical tube 201 to be inspected passes within the interior of the mirror. It is readily apparent that a sufficient space is left between the tube 201 to be inspected and the deflector 200 in order to ensure that the tube is capable of sliding freely inside the deflector.

The assemblies consisting of flat transducers and deflectors in accordance with the invention permit detection of longitudinal flaws in cylindrical parts with a much higher degree of sensitivity by means of longduration phasing of the Lamb waves with the rays of constant incidence. The deflector M also makes it possible to perform this phasing operation over a length which is independent of the generating-line on which the flaw is located. The relative movement of rotation of the part with respect to the transducer therefore no longer serves any useful purpose for the inspection of the entire part. Moreover, the deflector M can be chosen as a function of the angle of inclination -y of flaws with re spect to the axis of the tube and the value 5 y will accordingly be adopted.

By way of example, it is thus possible to employ two annular transducers each placed opposite to a deflector M designed in one case for transverse flaws and in the other case for longitudinal flaws. It is then only necessary to perform scanning by displacing the tubes in simple translational motion instead of the complicated and helical movements of the prior art.

These inspection and testing operations are of considerable value for observing thin cylindrical walls in which excitation by Lamb waves is particularly efficient. Continuous testing of metallic tubes by transducerdeflector units located outside the tubes for example are particularly well suited to the use of the devices in accordance with the invention. It is also possible to test solid rods within a certain volume in the vicinity of the surface.

Furthermore, in addition to the application to the generation of Lamb waves, the transducers in accordance with the invention as well as the mirrors and lenses described have a useful function in exciting waves to a predetermined depth from the surface of the body to be tested. In fact, iflongitudinal waves are projected onto the surface of a cylinder for example and the rays of said waves make a constant angle of incidence with the normal to the surface at the point of impact, the waves which are excited within the medium (transverse or longitudinal waves) exist only within the space formed between the external surface and the enclosed surface of the rays which are generated by refraction. The enclosed surface is parallel to the external surface. Similarly, in order to prevent the phenomena of time dispersion at the point of reception, it is an advantage to produce waves of constant incidence within a material of small thickness, said waves being propagated in zigzag paths between the two faces of the material.

What we claim is 1. A deflector for converting a beam F of rays parallel to the generating-lines of a cylindrical part C having a polygonal directrix into a beam F of rays impinging upon the surface of said part at a constant angle of incidence i so as to generate therein refracted waves in which the sections of the associated wave surfaces along the surface of said part are at right angles to a family of broken helices, each helix being constituted by a plurality of straight-line segments which are concurrent in pairs along the edges of the cylindrical part C, each segment being inclined at an angle [3 with respect to a plane at right angles to the generating-lines of said cylindrical part C, wherein said deflector is constituted by a plurality of flat deflecting elements, at least one deflecting element M, being associated with each face P; of said part, the orientation of each flat deflecting element M, being defined by the direction of its normal n,- whose projections are proportional to cos a on a generating-line of the part C, to cos j sin a on a normal to the face P, and to sin j sin a on an axis at right angles to the two projections aforesaid, the anglesj and a being defined from the angles 1' and B by the relations tanj tan i cos B and cos 204 i sin 1 sin B and wherein the planes of two flat deflecting elements M, and M associated with two consecutive faces P, and P of the part C are deduced from each other by means of a movement of rotation followed by a movement of translation, said movement of rotation being performed about an axis parallel to the generating-lines of the part C and located in a plane which is equidistant from the edges A, and A,,,, the edge A, being at the intersection of the faces P, and P, and the edge A being at the intersection of the faces P, and P, the angle of rotation being equal to that of the dihedron constituted by the faces P, and P,,,, and the movement of translation being performed in a direction parallel to the generating-lines by a quantity a sin j tan a, where a is the width of the face P,

2. A deflector according to claim 1 for converting a beam F of rays parallel to the generating-lines of a cylindrical part C having a polygonal directrix into a beam F of rays impinging upon the lateral surface of said part at a constant angle of incidence i and such that B with the result that the families of broken helices are reduced to the family of parallel directrices located in equatorial planes at right angles to the generating-lines, wherein said deflector is constituted by a plurality of flat deflecting elements, at least one deflecting element M, being associated with each face P, of said part, the section of each deflecting element M, aforesaid along a plane at right angles to the generating-lines of the part C being a straight-line segment S, which makes an angle of the same value 1' with the associated face P, and wherein two of said segments S, and S, forming part of two consecutive deflecting elements M, and M, are equidistant from the edge A, which is the intersection of the two faces P, and P,,,

3. A deflector according to claim 1 for converting a beam F of rays parallel to the axis of a cylindrical part C having a polygonal directrix into a beam F of rays impinging upon the surface of the part C at a constant angle of incidence i and such that B 1r/2, with the result that the families of broken helices are reduced to segments of generating-lines, wherein said deflector is constituted by a plurality of flat deflecting elements M, forming a pyramidal surface, each deflecting element M, associated with the face P, being such as to make an angle 14 with the associated face P,

4. A deflector according to claim 2, wherein the eumulated length of the segments S, is constant in each plane at right angles to the generating-lines.

5. A deflector for converting a beam F of rays parallel to the generating-lines of a cylindrical part C having a circular directrix of radius R into a beam F of rays impinging upon the surface of said part at a constant angle of incidence i so as to generate therein refracted waves in which the sections of the associated wave surfaces along the surface of said part are at right angles to a family of circular helices formed on said surface having a pitch 2 rrR tan B, the angle B being the angle between the tangents to the helix and the plane at right angles to the generating-lines of the part C, wherein the surface of the deflector is a portion of the surface which has the equation expressed in polar coordinates (p z c2t a: S=R= sin R sin 1,0,

the angles a and j being related to the angles i and B by the relations tan j tan i cos B and cos 2a= sin 1' sin B.

6. A deflector according to claim 5 for converting a beam F of rays parallel to the generating-lines of a cylindrical part C having an axis Oz and a circular directrix of radius R into a beam F of rays impinging upon the lateral surface of said part C at a constant angle of incidence i along a family of circles which are transverse cross-sections of the cylindrical part, wherein the surface of the 'deflector is a portion of the surface which has the equation expressed in polar coordinates of axis 02:

Z= VP2R2 sin i+R sinif) 7. A deflector according to claim 6, wherein the intersection of the surface of the deflector along the planes at right angles to the generating-lines are arcs of involutes of circles of constant length.

8. An application of the deflector according to claim 5 to the detection of flaws in a cylindrical part C by excitation of Lamb waves in said part of small thickness, wherein the deflector is associated with a flat transducer which emits ultrasonic rays parallel to the generating-lines of the part to be inspected and performs the function of an emitter-receiver.

9. An application of the deflector according to claim 8, wherein the deflector and the transducer are located inside the space delimited by a cylindrical tube constituting the part C and also filled with a couplant liquid.

10. An application of the deflector according to claim 8, wherein the deflector and the transducer are located outside the cylindrical tube constituting the part C and also wherein the complete assembly is immersed in a couplant liquid. 

1. A deflector for converting a beam F of rays parallel to the generating-lines of a cylindrical part C having a polygonal directrix into a beam F'' of rays impinging upon the surface of said part at a constant angle of incidence i so as to generate therein refracted waves in which the sections of the associated wave surfaces along the surface of said part are at right angles to a family of broken helices, each helix being constituted by a plurality of straight-line segments which are concurrent in pairs along the edges of the cylindrical part C, each segment being inclined at an angle Beta with respect to a plane at right angles to the generating-lines of said cylindrical part C, wherein said deflector is constituted by a plurality of flat deflecting elements, at least one deflecting element Mi being associated with each face Pi of said part, the orientation of each flat deflecting element Mi being defined by the direction of its normal ni whose projections are proportional to cos Alpha on a generating-line of the part C, to cos j sin Alpha on a normal to the face Pi and to sin j sin Alpha on an axis at right angles to the two projections aforesaid, the angles j and Alpha being defined from the angles i and Beta by the relations tan j tan i cos Beta and cos 2 Alpha + or - sin i sin Beta and wherein the planes of two flat deflecting elements Mi and Mi 1 associated with two consecutive faces Pi and Pi 1 of the part C are deduced from each other by means of a movement of rotation followed by a movement of translation, said movement of rotation being performed about an axis parallel to the generating-lines of the part C and located in a plane which is equidistant from the edges Ai and Ai 1, the edge Ai being at the intersection of the faces Pi 1 and Pi and the edge Ai 1 being at the intersection of the faces Pi and Pi 1, the angle of rotation being equal to that of the dihedron constituted by the faces Pi and Pi 1, and the movement of translation being performed in a direction parallel to the generating-lines by a quantity a sin j tan Alpha , where a is the width of the face Pi.
 2. A deflector according to claim 1 for converting a beam F of rays parallel to the generating-lines of a cylindrical part C having a polygonal directrix into a beam F'' of rays impinging upon the lateral surface of said part at a constant angle of incidence i and such that Beta 0 with the result that the families of broken helices are reduced to the family of parallel directrices located in equatorial planes at right angles to the generating-lines, wherein said deflector is constituted by a plurality of flat deflecting elements, at least one deflecting element Mi being associated with each face Pi of said part, the section of each deflecting element Mi aforesaid along a plane at right angles to the generating-lines of the part C being a straight-line segment Si which makes an angle of the same value i with the associated face Pi and wherein two of said segments Si and Si 1 forming part of two consecutive deflecting elements Mi and Mi 1 are equidistant from the edge Ai which is the intersection of the two faces Pi and Pi
 1. 3. A deflector according to claim 1 for converting a beam F of rays parallel to the axis of a cylindrical part C having a polygonal directrix into a beam F'' of rays impinging upon the surface of the part C at a constant angle of incidence i and such that Beta pi /2, with the result that the families of broken helices are reduced to segments of generating-lines, wherein said deflector is constituted by a plurality of flat deflecting elements Mi forming a pyramidal surface, each deflecting element Mi associated with the face Pi being such as to make an angle
 4. A deflector according to claim 2, wherein the cumulated length of the segments Si is constant in each plane at right angles to the generating-lines.
 5. A deflector for converting a beam F of rays parallel to the generating-lines of a cylindrical part C having a circular directrix of radius R into a beam F'' of rays impinging upon the surface of said part at a constant angle of incidence i so as to generate therein refracted waves in which the sections of the associated wave surfaces along the surface of said part are at right angles to a family of circular helices formed on said surface having a pitch 2 pi R tan Beta , the angle Beta being the angle between the tangents to the helix and the plane at right angles to the generating-lines of the part C, wherein the surface of the deflector is a portion of the surface which has the equation expressed in polar coordinates ( Rho , theta ) of axis Oz, z cotan Alpha square root Rho 2 - R2 sin j + R sin j, theta , the angles Alpha and j being related to the angles i and Beta by the relations tan j tan i cos Beta and cos 2 Alpha sin i sin Beta .
 6. A deflector according to claim 5 for converting a beam F of rays parallel to the generating-lines of a cylindrical part C having an axis Oz and a circular directrix of radius R into a beam F'' of rays impinging upon the lateral surface of said part C at a constant angle of incidence i along a family of circles which are transverse cross-sections of the cylindrical part, wherein the surface of the deflector is a portion of the surface which has the equation expressed in polar coordinates of axis Oz: z Square Root Rho 2 - R2 sin2 i + R sin I theta
 7. A deflector according to claim 6, wherein the intersection of the surface of the deflector along the planes at right angles to the generating-lines are arcs of involutes of circles of constant length.
 8. An application of the deflector according to claim 5 to the detection of flaws in a cylindrical part C by excitation of Lamb waves in said part of small thickness, wherein the deflector is associated with a flat transducer which emits ultrasonic rays parallel to the generating-lines of the part to be inspected and performs the function of an emitter-receiver.
 9. An application of the deflector according to claim 8, wherein the deflector and the transducer are located inside the space delimited by a cylindrical tube constituting the part C and also filled with a couplant liquid.
 10. An application of the deflector according to claim 8, wherein the deflector and the transducer are located outside the cylindrical tube constituting the part C and also wherein the complete assembly is immersed in a couplant liquid. 